The invention relates to the field of devices used to secure heavy objects to support structures, the objects having variable outer dimensions such as do cycling pressure vessels, more particularly to securing compressed natural gas (CNG) fuel tanks to vehicles.
Natural gas is more widely distributed throughout the world than crude oil. Since the 1920""s, natural gas has been used to fuel vehicles in such countries as Italy. In 1995, approximately 100,000 vehicles in the United States and half that in Canada were fueled by natural gas. Significant growth in natural gas-fueled vehicles, especially in fleets, is gradually occurring throughout the rest of the world.
Natural gas, as delivered through pipelines, cannot be used directly as a mobile vehicular fluid and must be first physically compressed, liquefied or adsorbed. Compressed natural gas (CNG) for use in CNG-fueled vehicles is compressed between 3,000 and 3600 psi (20.77-24.92 MPa) and stored in tanksxe2x80x94usually ferrous, aluminum or non-ferrous liners wrapped in a fiber-reinforced tensile composite.
National Fire Protection Association (NFPA) standards (See NFPA 52, Chapter 3xe2x80x943.3-3.9) require that CNG tanks must be secured to vehicles in such as a way as to prevent damage from road hazards, slippage, loosening or rotation, using a method capable of withstanding a static force, in the six principle directions (up, down, left, right, forwards, backwards), of eight times the weight of a fully pressurized container. For moving vehicles this equates to imposed relative acceleration or deceleration on the tank of 8 times gravity (8 g). Further, under these conditions, the tanks cannot be displaced more than xc2xd inch (13 mm).
When a vehicle accelerates or decelerates, the inertia of an attached tank resists the change in velocity, restrained to follow the vehicle""s change in velocity only by its mounting means. A rear-ending situation or head-on collision can impose significant forces on the mounting means. If they are not capable of restraining the tank, it could conceivably be sheared from the vehicle and become a projectile. The NFPA code anticipates that most such loads on a tank would not exceed 8 g and hence mounting means meeting the code are adequate.
Typically, for atmospheric or stationary tanks, the mounting means need only support the tank""s weight or restrain minor movement. However, wide changes in pressure (say 0 to 3000 psi) cause expansion and contraction of CNG fuel tanks. Typically, metallic liner CNG tanks can expand 0.5% at maximum fill pressure; a 500 mm diameter tank expanding about 2.0xc2x10.5 mm. Plastic liner tanks (such as those under Department of Transport (DOT) Federal Motor Vehicle Safety Standards FMVSS-304, NGV2-98 Std, Type 4 tank) can experience expansion of even 5 times that experienced with ferrous or aluminum liner tanks.
The restraining devices must be able to expand and contract with the tank through multiple tank expansion and contraction cycles. If they do not, then they lose the ability to continue to apply sufficient force to the tank after it contracts, resulting in non-compliance with the code and greater potential for prohibited and possible hazardous movement. Most recently, in Los Angeles, a transit vehicle actually lost a plastic-liner, composite CNG tank because the restraining device could not maintain a pre-load through the pressure cycles.
As CNG tanks are pressure vessels, they usually do not incorporate integrated mounting elements. Accordingly, and referring to prior retraining devices shown in FIGS. 1 and 2, FIG. 1 illustrates a mere continuous strap formed around a tank. It is simple but is incapable to dealing with variable tank dimensions. If initially fitted to a depressurized tank with sufficient load to secure the tank, a failure eventually or immediately occurs in the strap or fastener at the mounted end when the tank grows larger under pressure.
As shown in FIG. 2, conventional multi-piece steel straps and spring attachments are also used to secure CNG fuel tanks to vehicles. The springs are adaptive to the expansion and contraction, however, multi-piece construction can result in excess noise (important in vehicular use), variable strap load around the tank, and fatiguing of the connections between the strap and the spring. Therefore, straps using spring attachments must be tested and replaced more frequently and are associated with increased installation time and labor cost. Characteristic of spring attachments, the compression of the spring to its designed deflection is not necessarily representative of the load throughout the strap. The spring responds only to the resistance to load at the ends of the strap, permitting the unwary to apply a pre-load to the spring representing only the frictional resistance of the strap leaving the tangent of the tank, leaving large areas of the tank having no pre-load at all (shown as a gap of exaggerated proportions in FIG. 2).
U.S. Pat. No. 4,367,572 to Zielenski discloses a composite elastic strap for securing rectangular batteries in vehicles. This composite strap comprises an elongate, notched rubber member having a saw-tooth-shaped fabric reinforcement member embedded within it. The resilient rubber can be stretched to a maximum, governed by extension of the fabric reinforcement member. This non-metallic composite strap helps Zielenski meet the following objectives: avoiding application of a rigid holddown device to an otherwise fragile battery case (particularly at the right angle corners), avoiding placing conductive metal about an electrical device; and avoiding the associated corrosion common with batteries. The composite strap is fitted with plates and hooks at each end for attachment to the vehicle.
Zielenski""s application relies heavily on an elastomeric composite for achieving its forgiveness, anti-corrosion and electrical insulating properties. Unfortunately, the composite strap is neither capable of imparting the loads necessary, nor providing the long term consistent loading required for use with CNG tanks. Such a strap cannot withstand acceleration and deceleration and maintain secure attachment of large mass tanks to the vehicle. Substitution of a stronger metal reinforcement member defeats the forgiving and anticorrosion advantages. In particular, as stated by Zielenski, a metal restraint imposes unacceptably significant stresses on the upper edges of a battery. Additionally, in the CNG tank environment, this prior art strap will harden at low ambient temperatures, resulting in an inability to react to dimensional changes. As a tank shrinks, an initial pre-load will reduce or a space can even form between the strap and tank. The use of separate hooks for attachment further weakens the strap for restraining large loads.
There is therefore a demonstrated need for a restraining device that can reliably secure a heavy tank to a supporting structure regardless whether the tank is in an expanded or contracted state and is capable of maintaining its ability to retrain through multiple expansion and contraction cycles.
In one preferred aspect, an improved restraining device is provided for securely attaching heavy pressure vessels or tanks of curved section to the supporting structure of a vehicle. Each restraining device is a strap comprising a thin elongate unitary metallic member which, in use, extends about the curved wall of the tank, the ends of which are fastened rigidly to the supporting structure. The otherwise planer body of the strap has one or more localized bends. Axial loads impose on the straps due to dimensional changes to the tank are converted in part to moments in the bends, increasing the dimensional ability of the strap to lengthen without yielding. Accordingly, pressurization and depressurization of a CNG tank cause the strap to elastically lengthen and contract repeatedly while meeting or exceeding a predetermined restraining capability, without the use moving elements.
Therefore, in a broad apparatus aspect of the invention, an improved strap is provided for a restraining a tank, such as a pressure vessel, to a supporting structure. Pressure vessels are peculiar in that they have a contracted and an expanded statexe2x80x94affecting the size of the tank. Accordingly, the strap has an extensible, elongated body which is placed over the tank and has first and second ends. The improved strap further comprises means for fastening the first and second ends rigidly to the supporting structure and incorporates one or more V-bends formed in the body. The V-bends are spaced along the body between the first and second ends, each V-bend having an apex displaced out-of-line of the body so that when the tank is cycled between its contracted and expanded states, the body of the strap elastically shortens and lengthens while each V-bend flexes. When constructed from spring steel, the strap provides sufficient holding force to withstand acceleration and deceleration of the vehicle under normal operating conditions as well as under most impact conditions, whether the tank is fully expanded or fully contracted.
The improved strap makes it now possible to provide a novel method of securing pressure vessels to a supporting structure, such as a vehicle, comprising the steps of providing the above described strap and conforming it over at least a portion of the tank, the ends being rigidly attached to the supporting structure. So attached, a tensile stress is introduced into the strap""s body so that the tank is securely restrained to the structure, the body being capable of elastically moving between two extreme positions so that when the tank cycles between depressurized and pressurized states, the V-bends in the body elastically flex sufficiently to continue to securely restrain the tank.